This invention relates to corona electrodes operated in an atmosphere which includes a high percentage of particulate toner in suspension. In this situation there is a tendency for the particles to collect on the surface of the electrode and thereby prevent its proper functioning. When the electrode is in the form of a fine gauge wire, problems arise in its handling and cleaning because the wire can be very easily broken. In one form of electrostatic development a finely divided dry electroscopic pigment powder or toner, usually mixed with carrier particles, is brought into contact with the photoconductive surface, where the powder is deposited in a pattern corresponding to the electrostatic image to which it is held by electrostatic attraction.
Improved control and enhancement of the effectiveness of this development step can be brought about by introducing an appropriate electric field or bias at the point where development is taking place, and one efficacious way of bringing this about is to apply an electrostatic charge of the appropriate voltage and sign to the rear surface of the photoconductor as it passes through the development zone. Such treatment customarily employs a corona discharge device whose output element or electrode is a fine gauge wire stretched near the path of the photoconductor.
In this description the term "fine gauge" is intended to identify the corona wires normally used in electrostatic copiers. Because of size, power supply, and human factor restrictions, these coronas are normally operated at close spacing to the work, and at minimum power input and hence the electrode wires are required to be of minimum practical diameter to produce effective ionization. The design limitations relating to machine size dictate the use of wires between 0.002 inches and 0.005 inches in diameter, which, even when made of high strength materials, are necessarily delicate and easily broken. Wires substantially in the foregoing strength range are contemplated whenever the term "fine gauge" is used hereinafter.
One serious difficulty with the use of a fine gauge corona wire at the development location is the fact that, with the unavoidable concentration of toner powder in the air near the development station, a certain amount of powder becomes attracted to and forms a deposit on the surface of the wire. When this happens, the effective diameter of the wire is increased and in consequence it becomes too large to function reliably as a corona electrode, and the quality of the developed copies experiences progressive degradation.
Various attempts have been made to overcome this problem, but probably none have been as effective as merely removing the electrode assembly periodically and wiping the wire clean by hand. This, however, is a rather awkward procedure, and tends to be untidy and to soil the operator's hands with toner powder. More important than that, however, is the problem of the delicate character of the small diameter wire which is easily broken, especially if the cleaning step has been neglected too long and the toner has become caked on the wire so that forceful removal is required.